Such an analog-to-digital converter is known inter alia from: J. J. van der Kam "A digital `decimating` filter for analog-to-digital conversion of hi-fi audio signals", Philips Technical Review 42, No. 6/7, pp. 230-238, Apr. 1986, in particular FIG. 3 thereof. Bivalent or 1-bit sigma-delta modulation is a technique by means of which an analog signal is converted into a 1-bit digital signal by oversampling with the aid of quantization means having a deliberately low resolution and high quantization noise. By means of a 1-bit digital-to-analog converter the digital signal is reconverted into an analog feedback signal, which is subtracted from the analog input signal in a differential stage. The difference of the two signals is filtered in a low-pass loop filter and is applied to a clocked comparator operating as a 1-bit quantizer. The use of a sufficiently high loop gain for baseband frequencies of the analog signal ensures that in the 1-bit digital signal the quantization noise in the baseband is comparatively low at the expense of a comparatively higher quantization noise above the baseband. However, as described comprehensively in the above article, a digital decimating filter enables the noise above the baseband to be suppressed effectively and also enables the oversampled 1-bit signal to be converted into a multi-bit digital signal at the desired lower sampling rate. This analog-digital conversion technique is attractive in particular because only a simple decision circuit (comparator) is required for the actual quantization.
In the case of analog-digital conversion of more than one input signal the need arises to limit the number of analog-to-digital converters. In the analog-to-digital conversion of more than one input signal it is known per se to utilize the analog-to-digital converter in time-sharing, an analog multiplexer being arranged in series with the input and a digital demultiplexer being arranged in series with the output of the analog- to-digital converter. For the actual analog-digital conversion an analog-to-digital converter of any customary type may be used, such as flash, multistep or successive approximation convertors. These converters have in common that they supply the output signals directly with the desired digital word length and at the desired sampling rate. Also in this case the use of a 1-bit sigma-delta modulator would be desirable in view of its simplicity. However, this is not possible because of the time delay in the loop of the sigma-delta modulator. The 1-bit sigma-delta modulator output signal would then be subtracted from an analog input signal which does not correspond thereto, which would result in an inextricable crosstalk between the individual input signals.
U.S. Pat. No. 4,837,527 discloses a series arrangement of an oversampling sigma-delta modulator and a decimating network to which a stereo multiplex signal, formed by multiplexing the L and R audio signals, is applied. The stereo multiplex signal comprises a baseband stereo sum signal (L+R), a 19 kHz stereo pilot tone, and an amplitude modulated stereo difference signal (L-R) on an 38 kHz suppressed carrier wave. The decimating network then supplies the digital representation of the stereo multiplex signal, which must subsequently be demodulated digitally to form the digital L and R signals. However, this demodulation is problematic because the suppressed carrier wave must be regenerated.
Japan Patents Abstracts JP-A-2-95024 shows an analog-to-digital converter comprising a sigma-delta modulator and a multiplexer for the application of more than one input signal. A certain transition time is required before the sigma-delta modulator signal is a correct representation of the input signal. This transition time depends on the sampling rate of the sigma-delta modulator. After switching over to another input signal it is again necessary to wait for the expiry of a transition time. In this known analog-to-digital converter this transition time is minimized by initializing the sigma-delta modulator upon each change-over to another input signal. This procedure makes the system comparatively slow.